Magnetic recording and reproducing system



D. E. TAYLOR Nov. 9, 1954 I MAGNETIC RECORDING AND REPRODUQING SYSTEM- Filed Aug. 30, 1949 Kurt E24 INVENTOR DUI/6145f. 771,101?

604? Wm K ZM.

ATTO RNEYS United States Patent i-MAGNETIC RECGRDING AND REPRODUCKNG SYSTEM Douglas E.-Taylor, Southport,'Conn., assignor to Dictaphone Corporation, Bridgeport, Conn.

Application August 30, 1949, Serial No. 113,123

9' Ciaims. .(Cl. 179-1002) 'This invention relates to sound recording and repro- Lducing systems using magnetic records; and particularly to apparatus for simplifying and improving the opera- .tionof'such systems wherein a single translating unit for -head is vused both for recording and reproducing.

In some magnetic recording systems, the signals to be recorded areapplied to the magnetic record medium by means of a recording head and are reproduced therelfrom by a separatereproducing head. It is possible, however, to use a singlehead to perform both of these functions, thereby simplifying the apparatus and lowering its cost. Such single-head arrangements are particularly desirable indictating machines, because of the saving in space made possible by the use of a single head, and thereduction in manufacturing costs resulting .from simplification of the head-supporting and moving structure.

f-Some difiiculties areencountered, however, whena 'single head is used both for recording and reproducing. For example, in order to obtain improved magneticrecording characteristics, it is common practice to apply va high-frequency bias signal to the record medium duraingrecording simultaneously with the sound signalwhich is being recorded. Although the frequency of this signal is above the audible frequency range, when'sucha highfrequencybias method is used in a system having only a single head for recording and reproducing, noisy and dis- .-torted reproduction may result. vSuch noise and distortion is caused by residual magnetism in the head. When the same magnetic circuit or core is used both for recording and reproducing, the high-frequency bias signal is disconnected .from the head atthe same time that the head is switched from the recording circuit to the reproducing circuit. If the high-frequency bias signal at the instant ofiswitching is of such value that the core .of the head is magnetized thereby at that instant, the core'will be left partially magnetized, and this residual vmagnetism will remain in the corethroughout the subr-sequent reproduction. Even though this residual magnetism is small, it'may be of sufficient magnitude to in- .ztroduce a magnetic bias in the operation of the head ,as a reproducing unit causing it to operate on anon-linear portion of its magnetization characteristic and thus introducing distortion andnoise in the reproduced signal.

If the'magneticcore of the head is carefully and properly annealed, thev residual-magnetism retained-by the :core undernormal operating conditions may bezsolow that no particularly harmful effects will be observed. However, in quantity production, where cost=and speed of production are major considerations, it is difiicultto .be certain that the characteristics of each core assembled .into .a..recording head are satisfactory. Unsatisfactory operation may result because the core was improperly .annealed in thefirst instance, or because thexsubsequent working of the metal of the core, which to some extent necessarily accompanies handling and assembly, may have changed the characteristics of the core material so that optimum magnetic characteristics are ,not obtained.

Inaccordance with this invention, a recordingand reproducing. system is'provided, in which any residual magnetism is positively removed fromthe core each time the recordrreproducecontrol lever of themachine is manipulated to switch the machine from recordingto reproducing condition, this demagnetization being ac- ..complished in such manner that it is effectively vcom- ;p leted before actual reproduction takes-place,xand with- ...out requiring. any .additional manipulations .by theoperuproducing. is provided from a conventional rectifier-filter .power supply,:generally indicated in block form at 16,

ator. The present invention accordinglyzresides 'in apparatus for improving the operation of magnetic recordclrcuit lS employed for both recording andareproducing.

One aspect ofthe invention is concerned with preventing the retention of residual magnetsim in the magnetic .core when the recording bias signal is removed. Another aspect relates to features whereby the improved system is adapted particularly fordictating machines. Another feature of the invention relates to-apparatus whereby the eiliciency ofa coupling circuit, which transfers .the signals from the bias signal generator to the recording head, is. gradually attenuated-at the termination of the recording operationwhile the bias signal continues to be applied to the coupling circuit. Still-another aspect of the invention relates to a simplified switching arrangement for acomplishing the foregoing results.

vAdditional aspects and advantages of the invention will be in part pointed out, and in part apparent from a consideration of the following description considered in conjunction with the accompanying drawing, which shows, diagrammatically, the electrical circuits of a magnetic-record recording-reproducing system embodying the invention.

The circuit diagram shown in the drawings is such as .might be used in a magnetic-record type dictating machine, for example, the dictating machine described in .U. S. patent application Serial No.. 767,2l0, fi led August 7, 1947. In this machine a single head is utilized both for recording and reproducing.

.dicated in block form M4. The output signal from amplifier 4 is fed'into a power amplifier stage, generally indicated at '5, which further amplifies the signal and feeds it through leads 6 and 7 toa recording head, generally indicated at 8, which isarranged to record the :signal on a moving magnetic record medium 9.

During the recording operation a high-frequency bias signal, produced by a slgnal generator, generally indicated at 19, passes through a coupling stage, generally indicated at 11, whose purpose will be explained later, and

is applied to the recording head '8 simultaneously with thesignal to be recorded from power amplifier stage 5.

When the apparatus is switched from record to retproduce condition, the bias-signal source is disconnected,

in .a manner to be explained later, and the signal in- 'duced in heads, by the moving magnetic record 9 is fed,

.by a-lead 12, into ,a pre-amplifier stage, generally indi- ;cated at v13, which is .used only during reproduction because the relatively weak signal picked up by head '8 requires greater amplification than does the signal delivered by the microphone 2. The output signal from preamplifier 13 is fed through the'switching arrangement '3, amplifier 4, and the power amplifier stage 5, as in rexcording,.and the amplifiedsignal is applied to an output circuit, generally indicated at 14, which includes a loudspeaker. 15.

. Power for this'apparatus used forrecording and rewhich is connected through a switch current supply lines 18.

'A' plurality of switches .S-1, 8-2, 8-3, S-4, S'5, S-6, and 8-7,: provided to change the circuit from recording to reproducing condition, are shown in recording position. In practice, the switches are ganged together (either 17 to alternating 'mechanically or electrically) and are controlled by a manually operable lever (not .shown), such as that disclosed in theabove-mentionedapplication, which conditions .theimachine for:recording or reproducing.

The'rec'ording portion of the illustrated apparatus'will now -.be described in greater detail. 'One terminal of the hand microphone 2, which; is conveniently provided with a switch 19 (connections for'which are not shown) for starting'and stopping the 'machine, is connected to .a common ground circuit and .the other microphone terminal is connected :throu'gha resistor 20.and a con- .i.plmg condenser 2l to one end-ofta;potentiometer 22,

which serves as a volume level control during recording. The opposite end of the potentiometer 22 is connected to ground and its sliding contact 23 is connected to contact point 24 of switch S4, arm 25 of which is connected through a lead 26 to the input circuit of the audio amplifier 4.

Amplifier 4 is energized from the power supply 16, the positive terminal 27 of which is connected to the amplifier through a lead 23, the negative terminal 33 of the power supply 16 being connected thereto through the common ground circuit.

The output signal from amplifier 4 is fed through a coupling condenser 38 to a control grid 42 of a pentode amplifier tube 44 in the power amplifier stage 5, a ground return circuit being provided from grid 42 through a resistor 46. The cathode 48 of this tube is connected to ground through a bias resistor 52, the screen grid 54 is connected directly to the positive voltage lead 28, and the anode 56 is connected to lead 28 through a load circuit comprising a primary winding 58 of an audio-output transformer 62, connected in parallel with a stabilizing condenser 64. When the apparatus is in recording condition, as shown, the secondary winding 66 of the transformer 62 is connected through switch arm 68 and contact point 7 2 of switch S2 and a dummy load resistor 74 to ground.

The audio signal developed across the anode load circuit of power amplifier tube 44 is fed through a coupling condenser 76, a series resistor 78, and arm 82 and contact point 84 of switch 8-3 to the winding 86 of the recording-reproducing head 8, to induce correspondingly varying magnetic flux in its core 87.

"the high frequency signal generator, or oscillator, generally indicated at 16, which provides the high-frequency bias signal that is applied to the recording head 8 simultaneously with the audio signal, comprises an oscillator tube 94, the anode 96 of which is connected to one end of a parallel resonant circuit comprising an inductance 98 and a parallel-connected condenser 102. The opposite end of this resonant circuit is connected through a condenser 104 to the control grid 196 of tube 94, and to its cathode 108 through a resistor 112, the cathode being connected to ground through a resistor 114. Positive voltage is provided for oscillator circuit by a connection from a tap 116 on the coil 98, through a series resistor 122 and contact point 124 and arm 126 of switch 8-4 to the positive voltage supply lead 28. A condenser 127 is connected from the coil tap 116 to ground and, in conjunction with the resistor 122, forms a time-delay circuit whose function will be described later.

The signal produced by the signal generator 10 is applied from anode 96 of the oscillator tube 94 through a coupling condenser 128 to one end of a gain-control potentiometer 132, the opposite end of which is connected to ground. The adjustable contact 134 of the potentiometer 132 is connected to a control grid 136 of a triode amplifier tube 138 of the coupling stage 11. The cathode 142 of this tube is connected through the resistor 114 to ground in common with the cathode circuit of due oscillator tube 94. The anode 144 is connected through a plate load resistor 146, and contact point 147 and arm 148 of switch 8-5 to the positive voltage supply lead 28. A condenser 152 is connected between contact point 147 and ground, and in conjunction with the resistor 146 forms part of a second timedelay circuit whose function will be explained hereinafter.

The signal developed across load resistor 146 is upled from the anode 144 of tube 138 through a conldlensler8 154 to the winding 86 of the recording-reproducing The coupling stage 11 between the signal generator 10 and the head 8 can be made either to amplify or to attenuate the high frequency bias signal produced by signal generator 10, dependent upon the desired operating condition, by suitable adjustment of the contact 134 of the potentiometer 132. The function of this coupling stage will be explained more fully later.

When the control lever (not shown) is switched from the recording to the reproducing position, all of the switches S-1 through S7 are changed from the positions shown in the drawing to their alternate positions. The recording-reproducing head 8 now picks up the signal previously recorded on the magnetic record medium 9.

This signal, which is induced in the winding 86 of the head 8, is fed by the lead 12 and a coupling condenser 162 to a control grid 164 of a pentode vacuum tube 166 of the pre-amplifier stage 13, a grid return circuit being provided by a resistor 168 connected between grid 164 and ground. The cathode 170 of this tube is connected to ground through a bias resistor 172 and contact point 174 and arm 176 of the switch 8-6, which is provided to render this pro-amplifier stage inoperative during recording when it is not required. The suppressor grid 178 of pre-amplifier tube 166 is connected to the cathode 179, which is connected also to the positive supply lead 28 through a voltage-dropping resistor 182, and the screen grid 184 is connected to the positive supply lead 28 through a resistor 186 and is bypassed to the cathode 170 by a condenser 188. This pre-amplifier stage 13 is completed by a connection from anode 192 through a plateload resistor 194 to the positive voltage supply lead 28.

The amplified signal which appears on anode 192 is coupled through a condenser 196 to volume and tone control circuits, which comprise two parallel branches. The tone control circuit includes a condenser 198, a potentiometer 2112, and a condenser 204 connected in series between condenser 196 and ground. The volume control circuit includes a resistor 206 connected in series with a potentiometer 208 between condenser 196 and ground. The sliding contact 212 of the tone control potentiometer 262 is connected to the junction of resistor 206 and potentiorneter 208 in the volume control branch.

The sliding contact 214 of the reproducing volume control potentiometer 208 is connected to contact point 216 of switch S7 and to contact point 218 of switch Sl. When the apparatus is in recording position, the arm 222 of switch S7 connects the output of amplifier tube 192 to ground to prevent any noise, or other undesirable signals, from being impressed on the recording system. When the switches are in the reproducing position, arm 222 of switch S7 is connected to the contact point 224, grounding the signal from microphone 2, and the reproduced signal is applied through the contact point 218 and the arm 25 of the switch 5-1 to the input circuit of the audio-frequency amplifier 4.

The circuits associated with the output of amplifier 4 function as described above in connection with the recording operation, except that the signal from power amplifier tube 44 is applied to the voice coil 232 of the loudspeaker 15 through the arm 68 and contact point 234 of switch S2, and no signal is applied to the head 8 through condenser 76, because arm 82 of switch S3 is now connected to terminal 228, which is grounded.

The operation of the above-described system during the recording process is similar to the conventional systems heretofore known. However, when the control lever (not shown) is moved to the reproduce position, the magnetic core 87 of the recorder-producer head 8 is automatically and positively demagnetized. In order to accomplish this, the gain of the tube 138 is controlled, in this example, by gradually reducing its plate voltage so that the high-frequency bias signal appearing at anode 144 gradually decreases to zero.

When switch arm 148 of switch S5 is moved from contact point 147 to contact point 236 (simultaneously with corresponding movement of all of the switches 8-1 to S7), the plate voltage is not removed instantly from coupling tube 138, because of the condenser 152, which has been charged to the full potential of the positive voltage supply lead 28. However, as the condenser 152 discharges through the resistor 146 and tube 138, the anode potential, and accordingly the amplitude of the bias signal passing therethrough is gradually decreased. If this period of gradual attenuation occurs during the time required for 50 to cycles of the oscillator frequency satisfactory results are obtained under ordinary operating conditions.

If the signal generator 10 were caused to stop oscillating abruptly before the coupling circuit had sufliciently attenuated the signal, irregular wave forms of considerable magnitude might be produced by the signal generator at its cessation of oscillation which would interfere with proper demagnetization of the core 87 of head 8. Accordingly, the signal generator 10 is caused to stop oscillating only after the coupling has been suitably attenuated, by the gradual removal of plate voltage from the coupling tube 138. This is accomplished by providing the condenser 127 with a much larger capacity than m "and the condenseflISL-sothatwhen switch =arrn 126' o'fi switch J S-4 is moved. from terminal 124* to.terminal l5fl to' disconnect thesignal generator '"from the positive -voltage supply lead '28; the signal"generator--1 0 is 'caus'ed to continue oscillating for a period of i time a=substantially --'--longer than that required to discharge-condenser 152, an'd -transientsor unusual wave forms occurring at "the cessation of oscillations -in signal" generator 10, --=will'-not be impressed on winding 86, andthecore'Will accofdmgly be fully demagnetized by the gradual attenuation to'zero of-the bias signal applied thereto. The period -'o'f'time-"required for the demagnetization is, at most, a few hundred cycles, so that "the whole "operation "can -'take place in'assmall fractionof'a second. Thus, there is no-possibility that:interference between the recording and =reproducing circuits will be caused by 4 these "time --'delay circuits. In one typical embodimentof' the' invention, the condenser 152 had a "value of- 0105" micro farad with --a load'resistance 146 of "27,000 ohmsywhilefin' the -Signal generator'circuit 10, the condenser 127had a value of- 10.0 microfarads-and' the'resistor 122'had a value"of 1Q0,000 ohms.

. 'Thus, 'it' will-be seen that 'theabove-describedembddiment 'of this invention is well adapted to carryout the ends and'objects hereinbefore set forth. Althoughonly a single embodiment of 'the invention has been described, it is apparent that it'can'be'modified was to "bestsuit the needs of *aparticularuse, 'andthatcertain eatures of the invention can attimes be use'd'i'to advantage without a corresponding use of other features. It is to be understood, thereforeflthat the'example set forth above is for the-purpose of teachingthosefarriiliar -W1i.h this art the principles ofthe present invention and "how to apply them fortheir particular uses, andfisnot -1ntended to'be limiting of the invention or .an exhaustic analysis of all its-"possible forms or uses.

I claim:

1. Apparatus for "improving the quality .of the .reproduced signals a magnetic ..recor'ding.:and reproducing system including a recordinghead having a common magnetic circuit used bothforrecording and .reproduc ng, and an amplifiersystem coupled..to.:said mag- -netlc circuit for amplifying signals'when recording and when reproducing, said apparatus including a signal generator circuit producing a supersonic bias signaha coupllng circuit including an .electronicamplifier stage extending between said generator and said'head arranged'to nnpress said bias .signalon SaidtmagneticIcir- .cult, a source of potential .to energize .said amplifier stage, a switch arranged gization from said amplifier stage to render said couplmg circuit noperative, and energy storage "means arranged to maintain energization of said electronic amplifier with gradually decreasing energization voltage for a predetermined period whereby the transfer efficiency of said amplifier stage lisgradually reduced to zero as said energizationvoltage .1s reduced, .thus attenuating to zero the supersonic bias s gnal impressed on said magneticcircuit.

'2. Apparatus for improving .the quality of the reproduced s gnals in a magnetic recording and reproducing system lIlClLldlllg a recording head having a 'common .inagnetlc .clrcuit used both for recording and reproducmg, and an amplifier coupled to -said magnetic'circuit for amplifying signals to be recorded by said head and for amplifying signals picked-up by said head, said apparatus 1nc lud1ng switch means having a first position for conditioning said apparatus for recording and a second pos1t1or 1 for conditioning said apparatus for reproducing, a signal generator producing a supersonic bias signal, and a coupllng circuit including an, electronic amplifier stage extending from said generator to said head and being arranged to impress said bias signal on said magnetic circuit, a supply of energization potential eonnectable to said amplifier stage, said coupling circuit mcludmg energy storage means for temporarily maintaining the energization voltage of said amplifier stage, a second switch under the control of said switch means arranged to cut 01f the energization potential supply from said amplifier stage to enable the energization voltage from said energy storage means to decrease gradually the transfer efficiency of said coupling circuit effectively to zero value over a predetermined period.

3. In a magnetic recording and reproducing system F .to cut off .said zsource of enerafter said switch is actuated,

- the same magnetim circuit -used iialternatively & both for -recording and =-reproducing,

appa'ratus for improving the :quality of the I reproduced signalslincluding first switch :means 2 having a first/positionain which -.said apparatus is conditioned forrecording and a 1 second position .in which #said apparatus is conditioned for Jreproducing, .amelectronic signal genrerator for producing a -bias signal of frequency above the 1 recording 1 range utilized .r by .1 the system, a coupling eircuit including a an electronic amplifier \extendin g from I :said generator: to said head: arranged to impress saidi-bias signal on said magnetic circu-it, *second switch means under control of said-first switch means for deenergizing said generator and .the electronic amplifier 1 in I said -coupling circuit 1 when a said first switch means 2 is :chan'ged afrom its first .;position, and =energy storage means :for :maintaining energization 1 of :the electronic uamplifier -in said coupling circuit with ::gradually x decreasing energizting -voltage I for a ipredetermined z'period after said -Jgenerator. has been 'deenergized.

4. In a magnetic. recording and: reproducing systemaincluding- 'arecording-reproducingwheadshaving a common .magneticcircuit that isuse'd :bothrfora'recording andzretproducing, apparatus "for improving I the squality fof ithe xreproduced signals including an electronicasignal:generator for producing a high-frequency rbias signal :switch sociated with .said second-switch means-land forming ;part-of said coupling circuit for gradually decreasingwithe transfer efiiciency oflsaid couplingcircuit overa second predetermined period shorter than "said first .predeter- 1' mined period, and means :rendering said second "switch means and time delay .icircuit. operative .uponwactuation of said' switchmeans.

5. In an 'office type "dictating machine havingta 1 single head-in which "the same'magnetic circuit :is dlSedifOI "magnetically recording and "reproducing Jsound signals, apparatus for improvingthe quality of the :reproduced signals including a signal generator circuit for :producing high frequency bias 'signals, a coupling circuit 'connected from said generator tosaid head for couplingasaid bias signals to said 1 magnetic circuit, a'power source for energizing said generator and "coupling circuits, switch means having first and secondipositionsiforcondition-ing "said machine for" recording and reproducing, respectively, said switch means being arranged to 'Jdisconnectisaid "power 'source from said generator 'and coupling circuits when moved from said first position, first energy stor- "age means for maintaining operation "of said generator for a-firstpre'determined period after said switch'smeans is actuated from said first 1 position, and second "energy storage means'for maintainingoperation of said coupling circuitwith gradually increasing 'attenuationforza second predetermined period after said switch means-sis moved from said first position, said 'energy storage'ide' vices being' so proportioned #that rsaid second rperiodsis shorter than said first period.

6. In a dictating machine having a single head in which the same ferromagnetic circuit is used both for recording and for reproducing sound signals, apparatus for improving the quality of the reproduced signals including a vacuum tube oscillator circuit for generating a high frequency bias signal, a coupling stage comprising vacuum tube amplification means for coupling said bias signal from said generator to said magnetic circuit, manually adjustable means for regulating the gain of said coupling stage, power supply means for energizing said generator and said coupling stage, switch means having first and second positions in which said machine is conditioned for recording and for reproducing, respectively, said switch means being arranged to disconnect said power supply means simultaneously from said generator and from said coupling stage when moved from said first position, first energy storage means comprising a first condenser and discharge circuit for maintaining operation of said generator for a first predetermined period after said switch means is actuated from said first position, and second energy storage means comprising a second condenser and discharge circuit for maintaining operation or said coupling stage with gradually decreasing gain for a second predetermined period after said switch means is actuated irom said first position, said condensers and discharge circuits having values such that said second period 18 shorter than said first period, whereby when said switch means is moved from said first position, said coupling stage and said oscillator are sequentially rendered inefi'ective and residual magnetism in said magnetic circuit is reduced ett'ectively to zero value.

7. In a dictating machine having a single head with a common magnetic circuit used both during the recording and during the reproducing of sound signals, apparatus for improving the quality of the reproduced signals including an amplifier system coupled to said magnetic circuit for amplifying signals to be recorded and signals picked up by said head, a signal generator circuit for producing supersonic bias signals, a coupling stage comprising a vacuum tube amplifier for coupling said bias signals to said magnetic circuit, a power source for energizing said generator and coupling stage, switch means having first and second positions for conditioning said machine for recording and reproducing, respectively, said switch means being arranged to disconnect said power source from said generator and said coupling stage when moved from said first position, first energy storage means for maintaining operation of said generator for a first predetermined period after said switch means is actuated from said first position, and second energy storage means for maintaining operation of said coupling stage with gradually increasing attenuation for a second predetermined period after said switch means is actuated from said first position, said first and second energy storage means being so proportioned that said second period of time is at least equal to the period of fifty cycles of said bias signals and shorter than said first period, whereby when said switch means is moved from said first position, said coupling stage and said generator are sequentially rendered ineffective and residual magnetism in said magnetic circuit is reduced effectively to zero value.

8. In a dictating machine having a single head with a common magnetic circuit used both for recording and for reproducing sound signals, apparatus for eliminating the residual magnetism in said magnetic circuit each time said machine is changed from recording to reproducing condition, said apparatus including a signal generator for producing a supersonic bias signal, a coupling stage comprising a vacuum tube amplifier for coupling said bias signal to said magnetic circuit, a power source for energizing said signal generator and said coupling stage, switch means having first and second positions for condiitoning said machine for recording and reproducing, respectively, said switch means being arranged to disconnect said power source from said signal generator and from said coupling stage when moved from said first position, first energy storage means comprising a first condenser and charging and discharging circuits there for for maintaining operation of said generator for a first predetermined period after said switch means is actuated from said first position, and second energy stor' age means comprising a second capacitor and charging and discharging circuits therefor for maintaining operation of said coupling stage with gradually increasing attentuation for a second predetermined period after said switch means is actuated from said first position, said first and second capacitors and discharging circuits having values so proportioned that said second period of time is at least as long as the time required for 50 cycles of said bias signal and shorter than said first period, whereby when sard switch means is moved from said first position, said coupling stage and signal generator are sequentially rendered ineffective and residual magnetism inlsaid magnetic circuit is reduced efiectively to zero va ue.

9. In a dictating machine wherein sound signals are recorded on and reproduced from a magnetizable material, the combination including an audio amplifier having input and output circuits, a microphone, a recording and reproducing head having a magnetic core and a winding for inducing magnetic flux within said core, said winding being coupled to said audio amplifier, switch means having recording and reproducing positions, said switch means being arranged to couple said microphone to an input circuit of said audio amplifier and said winding to an output circuit of said amplifier when in the recording position, and to couple said winding to an input circuit of said amplifier and said microphone to an output of said amplifier when in the reproducing position, a supersonic bias-signal generator comprising a first vacuum tube having a cathode, an anode, and a control electrode, a bias-amplifier coupling stage comprising a second vacuum tube having a cathode, an anode, and a control electrode, said second vacuum tube having its control electrode coupled to said generator and its anode coupled to said winding, a source of power, a first circuit extending from said power source to said generator and including a first condenser arranged to be charged thereby, a second circuit extending from said power source to said bias-amplifier coupling stage and including a second condenser arranged to be charged thereby, said circuits being under the control of said switch means and arranged to be disconnected from said power source when said switch means is moved from the recording position, whereby said first condenser maintains the operation of said generator for a first predetermined period after movement of said switch means from said recording position, and said second condenser maintains operation of said bias-amplifier coupling stage with gradually decreasing gain for a second predetermined period after movement of said switch means from said recording position, the values of said condensers being such that said first predetermined period is longer than said second period, whereby said magnetic core is relieved of residual magnetism each time said switch means is moved from the recording position.

References Cit-ed in the file of this patent UNITED STATES PATENTS Number 

